This invention relates to a color picture reading system for use in reading a color picture from the original by utilizing a plurality of image sensors, and more particularly, to a color picture reading system which can avoid the erroneous reading of the color picture due to errors in relative mechanical positions of the image sensors.
In a multicolor reading apparatus for use in reading color pictures from the originals, a plurality of image pick-up elements are used to convert optical images of the same original in different ranges of wave length into electrical signals, respectively. The signals obtained are processed to form picture signals.
FIG. 1 shows a known arrangement of an optical system for a multicolor reading apparatus capable of reading two colors; red and black. In this apparatus, light from a pair of fluorescent lamps 2, 3 illuminates the original 1 and is reflected therefrom towards a half mirror 4. The light transmitted through the half mirror 4 is focused by a lens 5 onto a first image sensor 6. The light reflected by the half mirror 4 passes through a cyanic filter 7 which removes the red color therefrom. This light is focused by a lens 8 onto a second image sensor 9.
As shown in FIG. 2, the first and second image sensors 6 and 9 convert the exposed optical images into electrical signals 11 and 12, respectively. As one example the output picture signals may have 1728 bits per scanning line. The picture signals 11 and 12 are applied to an operational circuit 13 and subjected to predetermined processing, so that a first video signal 14 representing the picture information of red and a second video signal 15 representing the picture information of black are output from the circuit 13.
FIG. 3 shows a typical example of such operational circuit. The picture signal 11 from the first image sensor is led to a first comparator 16 and compared with reference voltage V.sub.1. As will be seen from FIG. 4a, the reference voltage V.sub.1 is selected to be lower than a white (background) level 11.sub.H and a red level 11.sub.R of the picture signal 11, so that the first comparator 16 outputs the second video signal 15 (FIG. 4b) in the form of a binary signal wherein the black picture information is represented by the low level of the signal. The picture signal 12 output from the second image sensor is led to a second comparator 17 and compared with reference voltage V.sub.2, selected to be slightly lower than the white level 12.sub.H. Since the picture signal 12 (FIG. 4c) includes almost no red component of wave length, the latter having been removed by the cyanic filter, there is obtained a binary signal 18 (FIG. 4d) in which the white picture information only assumes an H (high) level. Binary signal 18 is inverted by an inverter 19 and thereafter applied to an input terminal of a 2-input NAND circuit 21 together with the second video signal 15 led to the other input terminal thereof. As a result, the NAND circuit 21 issues from its output terminal the first video signal 14 in which the picture information of red only assumes the signal state of an L (low) level.
In color picture reading systems of the type described, ideal color separation will not always take place and, on some occasions, the boundary portion between different colors on the original may be erroneously separated into another false color. This results from errors in mechanically adjusted positions of the respective image sensors. More specifically, in the optical system shown in FIG. 1, as an example, if the first image sensor 6 and the second image sensor 9 do not precisely correspond with respect to the original in their output bit relationship, the two picture signals 11, 12 (FIGS. 5a, b) are subject to a shift in time by one or more bits. When there occurs a time shift between the picture signals 11 and 12, the operational circuit 13 performs the erroneous operation in connection with the shifted portions 22 to 25. As a result, in the case of employing the operational circuit as shown in FIG. 3, the shifted portion 22 at the boundary from black to white would be erroneously separated and, would appear as red.
Such error in color separation occurs normally by a degree of 1 to 2 bits in the main scanning direction and the sub-scanning direction. FIG. 6 shows one example of how noise (red in this case) occurs in the prior art, with the picture information varying along the main scanning direction in the sequence white, black and white. FIG. 7 shows one example of how noise (also red in this case) occurs in the prior art, with the picture information varying along the sub-scanning direction in the sequence white, black and white. In each example the main scanning direction is assumed to be the horizontal direction and the sub-scanning direction is assumed to be the vertical direction. The error resulting from misalignment, as indicated above, is shown on the right hand side in each of FIGS. 6 and 7. This error in color separation results in a remarkable reduction in quality of the reproduced picture appearing on a recording apparatus or a display screen.